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Kicking Out Pathogens in Exosomes

2015; Cell Press; Volume: 161; Issue: 6 Linguagem: Inglês

10.1016/j.cell.2015.05.040

1097-4172

Oksana A. Sergeeva, Gijs R. van den Brink,

Toxoplasma gondii Research Studies

Host-pathogen interactions involve a series of attacks and counter-attacks. Miao et al. show that, although some invading bacteria can take shelter in lysosomes by neutralizing their pH, this respite is temporary, as host cells can expel them in exosomes. Host-pathogen interactions involve a series of attacks and counter-attacks. Miao et al. show that, although some invading bacteria can take shelter in lysosomes by neutralizing their pH, this respite is temporary, as host cells can expel them in exosomes. Animal cells have developed sophisticated mechanisms to sequester and clear foreign invaders. Although immune cells have dedicated systems in place to deal with pathogens, all cells have the ability to protect themselves (Randow et al., 2013Randow F. MacMicking J.D. James L.C. Science. 2013; 340: 701-706Crossref PubMed Scopus (179) Google Scholar). Cell-autonomous defense against microbial pathogens primarily involves engulfing the invaders in an autophagosome, which then fuses with a lysosome, where the low pH assists enzymes to degrade its contents (Levine et al., 2011Levine B. Mizushima N. Virgin H.W. Nature. 2011; 469: 323-335Crossref PubMed Scopus (2415) Google Scholar). However, in the ongoing battle between hosts and pathogens, microbes have evolved mechanisms to circumvent host defenses and escape destruction, often by targeting autophagy proteins or regulators (Levine et al., 2011Levine B. Mizushima N. Virgin H.W. Nature. 2011; 469: 323-335Crossref PubMed Scopus (2415) Google Scholar). In this issue of Cell, Miao et al., 2015Miao Y. Li G. Zhang X. Xu H. Abraham S.N. Cell. 2015; 161 (this issue): 1306-1319Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar report that, when uropathogenic E. coli (UPEC) infect bladder epithelial cells (BECs), they instead avoid degradation by neutralizing the lysosome. However, BECs have evolved a counter-attack; they sense lysosome malfunctioning through the mucolipin TRP channel 3 (TRPML3) and expel the invading bacteria via exosomes (Miao et al., 2015Miao Y. Li G. Zhang X. Xu H. Abraham S.N. Cell. 2015; 161 (this issue): 1306-1319Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar). UPEC are the common cause of most urinary tract infections (UTIs). Previous work from the Abraham lab demonstrated that UPEC are able to permeate the bladder barrier by fusiform vesicles that infiltrate superficial BECs (Bishop et al., 2007Bishop B.L. Duncan M.J. Song J. Li G. Zaas D. Abraham S.N. Nat. Med. 2007; 13: 625-630Crossref PubMed Scopus (169) Google Scholar) (Figure 1; entry). These vesicles are naturally exocytosed to regulate bladder surface area when urine accumulates, but when they are taken back up into cells, UPEC slip inside them and hitch a ride into the BECs. In response, the host cells quickly expel most of the bacteria, a process initially thought to involve fusiform vesicles fusing back with the membrane and releasing the UPEC. Surprisingly, Miao et al., 2015Miao Y. Li G. Zhang X. Xu H. Abraham S.N. Cell. 2015; 161 (this issue): 1306-1319Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar now find that some extracellular UPEC are encapsulated in host membranes. To determine why this was the case, the authors traced the bacteria within host cells and found that autophagy, which normally destroys invading bacteria, is actually needed for cells to expel UPEC. Altogether, their data point to the following intriguing sequence of events (Figure 1; proposed mechanism): (1) rupture of the fusiform vesicle or cytosolic UPEC themselves trigger autophagy; (2) the autophagosome fuses with compartments of the late endocytic pathway, multivesicular bodies (MVB), leading to amphisome formation; (3) the amphisome keeps the inner autophagosomal membrane intact around the UPEC and allows this membrane to fuse with intraluminal vesicles (ILVs); (4) the amphisome fuses with a lysosome, forming an autolysosome; (5) UPEC neutralize the autolysosome; and (6) the neutralized autolysosome undergoes TRPML3-dependent calcium-induced fusion with the plasma membrane, releasing UPEC-containing exosomes. These findings raise a number of interesting questions. Upon fusion with MVBs, what prevents degradation of the inner autophagosomal membrane? It is possible that, already at this stage, the pathogen prevents acidification of this compartment. Subsequently, since preventing MVB biogenesis diminished release of UPEC-containing exosomes, UPEC appear to require some interaction with intraluminal vesicles (ILVs). The simplest mechanism would be fusion or interaction via a kiss-and-run mechanism between the UPEC-containing membrane and the vesicles of MVB origin. However, given that only 10%–15% of the extracellular UPEC carry ILV markers, one possibility is that two different pathways enable UPEC to end up in host-membrane encapsulated vesicles. The first would be through the proposed autophagy-based route, and the second would be through direct incorporation into ILVs during their biogenesis. ILVs form from invagination of the limiting endosomal membrane, so it is indeed conceivable that UPEC free in the cytoplasm could be incorporated into nascent ILVs (Figure 1; alternative mechanism). Functional MVBs would actually be needed for both routes because they are necessary for autophagy-based pathways as well (Fader and Colombo, 2009Fader C.M. Colombo M.I. Cell Death Differ. 2009; 16: 70-78Crossref PubMed Scopus (329) Google Scholar, Filimonenko et al., 2007Filimonenko M. Stuffers S. Raiborg C. Yamamoto A. Malerød L. Fisher E.M. Isaacs A. Brech A. Stenmark H. Simonsen A. J. Cell Biol. 2007; 179: 485-500Crossref PubMed Scopus (499) Google Scholar). Distinguishing between single or multiple pathways to exosomal release will require imaging of individual UPEC through the entire process and determining whether all bacteria followed a similar route, a major technical challenge. In addition, how do bacteria neutralize the lysosomes and why does the host cell expel these malfunctioning lysosomes rather than fusing them with healthy lysosomes? Is this a physiologically occurring event hijacked by bacteria, or is it pathogen triggered? Canonically, exosomes are released when MVBs, rather than lysosomes, fuse with the plasma membrane. Recent research has shown that exosome release from early or late MVBs involves different regulators and contents (Colombo et al., 2014Colombo M. Raposo G. Théry C. Annu. Rev. Cell Dev. Biol. 2014; 30: 255-289Crossref PubMed Scopus (3589) Google Scholar). While we know that the UPEC-containing exosome release is calcium mediated, we do not know other aspects of its regulation, which could be significantly different than MVB-derived exosome secretion. BECs are uniquely positioned in that they can eject exosomes into urine that is then flushed away, leading to clearing of the pathogen from the host. Yet, chronic UTIs somehow persist, so this mechanism is likely not the end of the story. In this sequence of attacks and counter-attacks between host and pathogen, UPEC appear at first to be the stealthy victors, managing to infiltrate into host cells through the host's own fusiform vesicles. However, BECs launch a counter-offensive by sequestering UPEC in autophagosomes/MVBs, which would seem to destine the invaders for destruction. UPEC respond by disabling the BECs' weapons from the inside, neutralizing the autolysosome to avoid degradation; BECs then kick UPEC out by lysosome exocytosis. Once the UPEC are in exosomes outside the host cells, is the fight over, or do these bacteria have yet more tricks in the bag? For instance, might they take advantage of exosomes to spread to nearby cells? In addition, it isn't clear whether UPEC-loaded exosome release occurs only from the apical side of polarized BECs (Toops and Lakkaraju, 2013Toops K.A. Lakkaraju A. Commun. Integr. Biol. 2013; 6: e24474Crossref PubMed Scopus (5) Google Scholar). It will be interesting to find out whether BECs can also expel exosomes basolaterally toward deeper cell layers, where the infection could spread or persist. A TRP Channel Senses Lysosome Neutralization by Pathogens to Trigger Their ExpulsionMiao et al.CellMay 28, 2015In BriefNon-lytic expulsion of bacteria from infected cells is a powerful cell-autonomous defense strategy to rapidly reduce infection burden in the bladder. The expulsion from infected BECs is triggered by TRPML3. This lysosomal TRP channel senses the UPEC-mediated lysosome neutralization and releases Ca2+, triggering lysosome exocytosis to expel the bacteria. Full-Text PDF Open Archive